As United Kingdom Patent Nos. GB 2,069,176 and GB 2,138,162, and Japanese Patent Laid Open Publication No. 63-208021 have disclosed, it has been suggested that multibeam simultaneous exposure at an apparently narrow pitch can be carried out by tilting a linear laser diode array which comprises light emitting sources arranged in a line at comparatively long intervals (for example, approximately 100 .mu.m).
However, such a conventional multibeam scanning optical apparatus which employs a linear laser diode array has the following problems: as the number of light emitting sources increases, a collimator lens with a large effective diameter and a large field of view becomes necessary, which necessitates correction of aberration; and since the angle of view when laser beams are emergent from the collimator lens is large, the dots on a scanning surface after a scanning lens shift in a main scanning direction.
Recently, Photonics Research Incorporated in the U.S.A. developed a surface-emitting laser diode array (trade name: LASE-ARRAY) which is a different type from conventional edge-emitting type linear laser diode arrays. The surface-emitting laser diode array comprises light emitting sources which are arranged at equal intervals in the length direction and in the width direction. Therefore, by using this laser diode array, simultaneous printing of more lines becomes possible without enlarging the effective diameter of the collimator lens.
For example, if the surface-emitting laser diode array has nine light emitting sources which are arranged in three lines in the length direction and in three lines in the width direction at equal intervals of P.sub.4, the collimator lens must have an effective diameter of 2.times.(2).sup.1/2 P.sub.4. Compared with a conventional case using an edge-emitting linear laser diode array, the effective diameter of the collimator lens is reduced by approximately 35%. This also inhibits the aberration of the collimator lens. Also, the angle of view when laser beams are emergent from the collimator lens becomes small, and shifts of the dots on the scanning surface in the main scanning direction become small. However, an attempt at simultaneous printing of a plurality of lines by fully using all the light emitting sources of the surface-emitting laser diode array has not been made.
Meanwhile, in a laser beam scanning optical apparatus, generally, the magnification ratio in the main scanning direction is different from that in the sub scanning direction. If a surface-emitting laser diode array which has light emitting sources arranged in two directions at equal intervals is used in such an optical apparatus, the intervals among the projected laser beams in the main scanning direction and those in the sub scanning direction are significantly different. For example, if a surface-emitting laser diode array which has 10 light emitting sources in each line parallel to the main scanning direction and 10 light emitting sources in each line parallel to the sub scanning direction at equal intervals of dozens .mu.m is used in a scanning optical apparatus of which magnification ratio in the main scanning direction is approximately 10 and of which magnification ratio in the sub scanning direction is approximately 1, the intervals among the projected laser beams in the main scanning direction are approximately 10 mm, and those in the sub scanning direction are approximately 1 mm. Thus, the intervals among the projected laser beams in the main scanning direction and those in the sub scanning direction are significantly different.
When the intervals among the projected laser beams in the main scanning direction are large like in such a case, the function of the scanning lens to correct distortion is not sufficiently effective, and eclipses of the laser beams may occur and lower the picture quality. Further, when the intervals among the projected laser beams in the main scanning direction and those in the sub scanning direction are significantly different, design of lenses is narrowly limited, and positioning of the lenses is complicated.